[9-22] E3M7: The Haunted Halls (Secret Level)

Past the area with the yellow armor, a lift will take you up. Instead of going up, jump back off the lift and walk into the hole underneath it to find 100 Health.

In the room with the yellow armor, just before the area with the vore, look for two grates on the right. One has a small landing in front of it. Shoot this landing to reveal Quad Damage, which you will need for the vore.

After running through the nail trap, press the button on the wall to your left to lower an elevator. Look above the ceiling; there is a small Q button you can shoot to open a door revealing armor and Quad Damage.

After running through the nail trap, press the button on the wall to your left to lower an elevator. Go up to the bridge on the upper level and look down to the right. You should see a cage. Jump on top of this cage, and a door will open in front of you leading to the Invulnerability power-up.